In a number of processes, such as the refining of crude oil, the purification of natural gas and the production of synthesis gas from, for example, fossil fuels, sulfur containing gas, in particular H.sub.2 S containing gas, is released. On account of its high toxicity and its smell, the emission of H.sub.2 S is not desirable. A number of processes directed to the removal of hydrogen sulfide from gases are known.
In some of these processes, hydrogen sulfide is first concentrated by means of a liquid absorbent, whereafter the concentrated hydrogen sulfide gas is converted into elemental sulfur. In certain cases, it is possible to omit the first step, i.e., concentrating the hydrogen sulfide, and to convert it directly to elemental sulfur. In other cases, particularly in cases with relatively low H.sub.2 S concentrations and higher CO.sub.2 concentrations, frequently two or more concentration or separation steps are needed to produce sufficiently high hydrogen sulfide concentrations to allow economical conversion of hydrogen sulfide to elemental sulfur.
One of the best known methods of converting hydrogen sulfide to elemental sulfur is the so-called Claus process. In the Claus process, elemental sulfur is produced by reacting H.sub.2 S and SO.sub.2 in the presence of a catalyst. The Claus system uses a combustion chamber which, at 950.degree. C.-1,350.degree. C., converts 50 to 70% of sulfur contained in the feed gas into elemental sulfur. Sulfur is condensed by cooling the reaction gas to a temperature below the dew point of sulfur, after which the remaining gas is heated and further reacted over a catalyst. Normally, the gas passes through at least two such Claus catalyst stages.
The different stages of the Claus process may be represented by the following equations: EQU H.sub.2 S+3/2 O.sub.2 .fwdarw.SO.sub.2 +H.sub.2 O (I) EQU 2 H.sub.2 S+SO.sub.2 .fwdarw.S.sub.n +2 H.sub.2 O (II)
The overall reaction is: EQU 3H.sub.2 S+3O.sub.2 .fwdarw.3S.sub.n +3H.sub.2 O (III)
Below 500.degree. C., the symbol n has a value of approximately 8.
The final Claus exhaust gas still contains small amounts of H.sub.2 S, SO.sub.2, CS.sub.2, COS, CO, and elemental sulfur in the form of a vapor or mist. The exhaust gas can be subjected to post-combustion to convert substantially all sulfur species to sulfur oxides, for example, SO.sub.2 and SO.sub.3, which are then emitted into the atmosphere.
Sulfur emitted as sulfur oxides ("SO.sub.x ") into the atmosphere with the exhaust gas may amount to 2-6% of the sulfur contained in the feed gas in the form of H.sub.2 S. In view of air pollution and the loss of sulfur involved, further purification is desirable and is frequently mandated.
Claus aftertreatments have been developed. These are carried out after the last Claus stage or after the post-combustion. These aftertreatments include, for example, dry and liquid phase processes for catalytic conversion of H.sub.2 S and SO.sub.2 to elemental sulfur, catalytic hydrogenation and hydrolysis of sulfur compounds into H.sub.2 S for further processing, and oxidation of all sulfur compounds into SO.sub.x for further processing by sorption in dry processes or in wet processes.
Commonly owned U.S. Pat. No. 5,299,091 (Buchanan et al.) herein incorporated by reference, discloses the use of MOST catalyst or sorbent following the Claus tail-gas desulfurization process.
Commonly owned U.S. Pat. No. 5,514,351 is directed to desulfurizing tail gas from sulfur recovery units using sorbents.
It is desired to develop a process that is effective for conversion of hydrogen sulfide to elemental sulfur without the need for the use of the Claus process and any associated processes, such as a tail gas treatment process and possibly an acid gas enrichment process, although they may be optionally added.